I have a asus p5vdc-mx mb

This board only supports DDR400 and DDR2-533

See this link:
http://dlsvr03.asus.com/pub/ASUS/mb/socket775/P5VDC-MX/E2545_P5VDC-MX.pdf

your board might not accept mixed memory try same i would go ddr1 since that is original setup

The P5VDC2-MX motherboard specs state that DDR2 533/400 ram is supported. Your 800 is just too fast and is forced to operate at a slow speed. It is not unusual to get half the amount of memory shown when operating at half speed.

Yes this motherboard supports up to 2 GUG of RAM and this are the choices of RAM speed that you can get 533 MHz • 400 MHz • 333 MHz • 266 MHz

Helo frnd i think u should check the fsb of ram is compatible with ur mobo then try to install a compatible ram

Number of Memory Slots: 2×184pin + 2×240pin
Memory Standard: DDR 400/DDR2 533
Maximum Memory Supported:2GB

PC6400(DDR2 800) is not supported by your motherboard. Also max memory is 2GB. PC4200 is the fastest memory you can install. There might be a BIOS update available from ASUS website to increase the speed but you would have to check the site.

Remove the memory card in the motherboard and then clean the pins of the memory card by an eraser of a pencil and then plug it again. Start the computer and see if it works.

with rams, if the memory it has is higher than that of your cpu, the pc will not boot up or if it is too low compared to your cpu again.

thus, you say it has got two slots right, remove both of the 512's and insert the 1gig only, run your computer, i am sure it will boot up nicely. switch it off. put one 512, if it does not boot then you should know that your ram has an excess of memory that your cpu could not apprehend to.

post a coment if this does not work.

thanks,

wonder

Your mobo is quite old and its chipset can only support a maximum of 512MB PER SIDE (technically called a bank) of a memory module. I would guess that your 1gb memory module has 8 chips that are all located on one side of the module, putting all of the 1 GB on one bank. Thus, only 512MB of your 1 gb RAM is detected and usable by your motherboard (the remaining 32 mb is most likely being used by your onboard video). You'll need to find a double sided 1 gb memory module (usually 8 chips on each side).

Asus site says the motherboard supports DDR or DDR2 RAM up to a max of 2GB.

http://www.asus.com/products.aspx?l1=3&l2=11&l3=0&model=814&modelmenu=1